Drilling through the earth is a complex endeavor, requiring sophisticated drilling equipment and drilling fluids, commonly referenced as drilling mud, in order to drill substantial distances through the various types of rock and soil commonly encountered during the drilling of vertical and horizontal bores.
Drilling mud is generally pumped from a reservoir of the drilling mud to the bottom of the bore through a central channel in the drill string. Drilling muds must be capable of performing a number of different functions, including specifically, but not exclusively, (i) cooling and lubricating the drill bit, (ii) delivering hydraulic power to the drill bit, (iii) carry cuttings upward during circulation, (iv) suspending cuttings and preventing appreciable settling when circulation is stopped, (v) preventing blowout, (vi) prevent wash out, and (vii) preventing excessive permeation of water from the bore into the surrounding formations.
A number of different drilling muds have been used over the years. Simple, unadulterated water can be used for some limited applications, but is not generally used due to its limited functionality as a drilling mud. One of the most elementary drilling muds is water mixed with drilled solids, commonly referenced as native drilling mud. Native drilling muds are generally effective for cooling and lubricating the drill bit, delivering hydraulic power to the drill bit, and preventing blowout at shallow depths. However, native drilling mud possesses a limited ability to carry cuttings upward during circulation, suspend cuttings when circulation is stopped, or reduce permeation of water into surrounding formations and washout. Over time, a number of different additives have been developed for native drilling muds in an effort to overcome these limitations.
The standard base drilling mud used today in both vertical and horizontal drilling is an aqueous slurry of bentonite clay (hereinafter "bentonite mud"). The bentonite clay may be modified with any of a number of different organic and inorganic drilling fluid additives such as sodium hydroxide, barium sulfate, lignite, polyacrylamide, and/or carboxymethylhydroxyethyl cellulose in an effort to enhance performance of the drilling mud.
U.S. Pat. No. 4,867,256, issued to Snead, discloses a method and apparatus for introducing polymer additives, such as a polyacrylamide, into drilling muds by periodically pumping liquid polymer into the conduit used to convey drilling mud from a mud pit to the kelley of a conventional drilling rig. Snead specifically discloses that a convenient method for adding the polymer into the drilling mud is to inject a defined quantity of the polymer each time an additional length of pipe is added to the drill string. While this method of introducing additives into a drilling mud comprises a substantial advance over prior procedures, the method has limited potential for commercial application due to the potential variability in the concentration of polymer within the drilling mud, and the relatively high cost of most additives compared to the cost of standard bentonite mud. In other words, the performance benefits provided by addition of a drilling fluid additive by this method are generally considered insufficient to justify the significant increase in cost.
In order to provide drilling fluids having the appropriate concentration of bentonite and/or additives, drilling fluids are routinely prepared by a batch process wherein the bentonite and/or additives are mixed into a known volume of water and then pumped into the bore. Such a procedure is highly labor intensive and requires either multiple mixing tanks or periodic pauses in drilling to permit another batch of drilling fluid to be mixed.
Accordingly, a substantial need exists for a cost effective method of introducing drilling fluid additives into drilling mud.